The object of the work is to correlate changes in the electrophysiological properties, ultrastructure and acetyl choline content of neuromuscular junctions during prolonged indirect stimulation or following treatment of the junctions with black widow spider venom. Our past work on the electrophysiology and ultrastructure of frog neuromuscular junctions has shown that: a) Black widow spider venom depletes nerve terminals of their vesicles and also of their transmitter; b) Indirect stimulation at low frequencies (2/sec) in curare plus hemicholinium depletes terminals of their transmitter but not of their vesicles; c) Vesicles fuse with and reform from the axolemma and neither the fusion nor the reformation process is dependent on the synthesis of transmitter. In the future we plan: a) To begin fractionation of the spider venom to purify the component that causes depletion of vesicles; b) To continue our electrophysiological work and to deplete terminals of transmitter without using drugs like curare or hemicholinium, which have strong post synaptic effects, to determine whether the size of a quantum of transmitter (amplitude of miniature end-plate potentials) changes during stimulation to exhaustion; c) To begin chemical analysis by gas chromatography of the acetyl choline content of our preparations to corroborate the conclusions drawn from our electrophysiological analysis; d) To computerize the analysis of our electrophysiological data, especially to develop a program to measure the frequency of miniature end-plate potentials when the frequency is high (greater than 100/sec).